Expansion joints and decoupling membranes

26 Jun 2010
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United Kingdom
I am getting a floor tiled in my house. It’s 900x900x10 porcelain on a solid concrete floor. It is a big floor at 8m x 7m. 3/4 of it is a new extension with wet UFH. The other 1/4 is the old house with electric mat UFH.

I am a bit concerned about expansion and possibly slight movement between old and new. Was planning to use decoupling membrane across the whole floor but should I also have a movement joint? I don’t want to as it will look bad.

I will be getting a tiler to do this but would be great to have your opinion as well as his.

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The new floor is solid - is the old floor solid or suspended?
Was a wall knocked thro to open up for the new extension?
Can you post a pic of the join in the two floors?
Hello Bobsad
An external wall was knocked down between the new and old floors. The new screed now continues over where the old wall was. Solid concrete floors to old and new.

So, your new slab butted up to the old slab just the inside of the old external wall?
How did the new slab meet the old slab - was it just a straight pour or were any other arrangements made?
Does the new slab have insulation around its edges?
Are both slabs of the same depth - eg. 150mm?
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New slab is standard 100mm concrete on hardcore. Screed does have insulation around he perimeter but only on the external walls, not between new and old floors. The builder has cast the new slab up to the old wall that has been removed, not over it. Only screed over the old wall.

Not sure how thick the old slab is but it’s 70 years old. It has no normal screed. It is concrete, 15mm marmox glued down, 10mm SLC.

I assume that by "screed" you mean a semi-dry mix of sand and cement?
By "only screed over the old wall" that you mean the screed is filling the gap between the old slab and the new slab above the remains of the old wall?

I've asked a few questions about this wall and the join because thats where cracks might possibly later appear.

Given the info so far, and no photo, & your working with a square layout of tile then I'd say your best bet might be to use a 6mm caulked joint instead of grout. Caulk the grout line as you cross the remains of the old wall?

Things to consider:
the new slab and screed must be dry.
Schluter make great movement joints.

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